The present invention relates to an exposure method used when producing a photomask used for producing a semiconductor integrated circuit, a liquid crystal display device, a thin film magnetic head, or another microdevice by photolithography and an exposure apparatus for working the exposure method.
In photolithography process of a semiconductor device, a pattern of a photomask is transferred on to a wafer or glass plate coated with a photoresist (hereinafter also called a xe2x80x9cphotosensitive substratexe2x80x9d). As this type of projection exposure apparatus, wide use has been used in the past of a step-and-repeat type exposure apparatus (stepper). This step-and-repeat type exposure apparatus exposes and transfers a pattern of a photomask by reduction projection on individual shot areas of the wafer. Therefore, when exposure of one shot area ends, the wafer is moved for exposure of the next shot area. This process is successively repeated.
Further, to increase the range of exposure of the mask pattern, a step-and-scan type exposure apparatus (scanning stepper) has been developed which synchronously moves the mask and wafer in scanning motion with the projection optical system in a state of restricting the exposure light from the illumination system into a slit shape (for example, rectangular shape) and projecting reduction image of a part of a mask pattern using the slit light. This step-and-scan type exposure apparatus (scanning stepper) has the advantages of an aligner transfer method of transferring the pattern of the entire surface of the mask to the entire surface of the wafer at an equal magnification by a single scan exposure and the advantages of the stepper transfer method. Note that a photomask used in a step-and-repeat type or step-and-scan type reduction projection type exposure apparatus is also called a xe2x80x9creticlexe2x80x9d.
The photomask used in such an exposure apparatus has conventionally been produced by drawing a master pattern on a photomask substrate using an electron beam lithography system or a laser beam lithography, system. That is, a mask material is formed on the substrate, a resist is coated on it, then the master pattern is drawn using an electron beam lithography system or laser beam lithography system. Next, the resist is developed and etched etc. to form the master pattern by the mask material. In this case, if the magnification rate of the reduction projection type exposure apparatus using this photomask is 1/xcex2, the master pattern drawn on the photomask may be the pattern of the device enlarged xcex2-fold, therefore the drawing error due to the lithography system is reduced to about 1/xcex2 on the device. Therefore, it becomes possible to form the pattern of the device by a resolving power of about 1/xcex2 of the resolving power of the lithography system.
As explained above, in the past, the master pattern-of the photomask has been drawn by an electron beam lithography system or laser beam lithography system. These lithography systems draw master patterns directly based on the drawing data from a control computer. Recent LSIs and other devices, however, have become larger in size and improved in fineness and integration, so the master pattern of the photomask required for exposure also becomes larger in area and finer. Further, as the photomask, use is also made of a reticle for double exposure provided with a correction pattern for preventing transfer of unnecessary patterns, a so-called phase shift reticle provided with a phase shifter between adjoining patterns, etc. With these special photomasks, however, the amount of the drawing data tends to become greater than that of other photomasks. Due to this, the amount of drawing data required in an exposure apparatus for producing a photomask becomes massive.
Therefore, the drawing time required for drawing a master pattern of a photomask by such a lithography system has recently grown from 10 hours to around 24 hours. This increase in the drawing time is becoming a factor behind the rising cost of manufacture of a photomask.
In this regard, in an electron beam lithography system, it is necessary to correct the proximity effect caused by the back scattering distinctive to an electron beam. Further, it is necessary to correct the uneven electric field around the substrate due to the charging of the surface of the substrate. Therefore, to draw a master pattern as designed, it is necessary to measure the error of the drawing position etc. in advance under various conditions and make complicated corrections at a high accuracy and stability at the time of drawing. Making such complicated corrections during an extremely long drawing time such as the above with a high accuracy and stability on a continuous basis, however, is difficult. The disadvantage arises of drift of the drawing position during the drawing. Further, it is possible to suspend the drawing for calibration, but this has the disadvantage of the overall drawing time becoming even longer.
Further, the resolving power and other characteristics of the resist for electron beam use have not been improved that much. No rapid improvement in these characteristics is expected in the future as well. Therefore, if the pattern rule of semiconductor devices becomes finer in the future, the drawing time for the master pattern of a photomask is liable to become too long and the resolving power of the electron beam resist is liable to approach its limit making the required drawing accuracy impossible to obtain. Further, the amount of the drawing data in the control computer is also becoming massive to-the extent of difficulty for use in a single drawing operation.
A laser beam lithography system draws a master pattern using an ultraviolet band laser beam. There are the advantages that it is possible to use a resist giving a higher resolving power compared with an electron beam lithography system and that there is no proximity effect due to back scattering. The resolving power of a laser beam lithography system is inferior to that of an electron beam lithography system, however. Further, in a laser beam lithography system, since a master pattern is drawn directly in this system, the amount of drawing data becomes massive and data processing becomes difficult. Further, the drawing time becomes extremely long. Therefore, the required drawing accuracy is liable to not be able to be obtained due to drift of the drawing position etc.
To solve the above problem, the present assignee previously proposed an apparatus which enlarges the pattern for transfer, divides the pattern into a plurality of patterns of master masks, and successively projects and exposes images of the plurality of patterns of the mask patterns reduced by the projection optical system on the surface of the mask substrate (blank) while stitching them (hereinafter sometimes called a xe2x80x9creticle exposure apparatusxe2x80x9d or a xe2x80x9cmask exposure apparatusxe2x80x9d).
When producing a photomask used for the production of a microdevice (working mask) using a reticle exposure apparatus, a thin film of a mask material is formed on a mask substrate as the photomask substrate and a resist or other photosensitive material is coated on it. Next, reduced images of the plurality of patterns of master masks are transferred to the photosensitive material for example by an optical type reduction projection exposure apparatus by the step-and-repeat system or the step-and-scan system. By etching using the pattern of the remaining photosensitive material as a mask, a desired pattern for transfer (master pattern) is formed.
At this time, if the magnification rate of a for example optical type exposure apparatus for producing a photomask is made 1/xcex1 (where a is an integer or fraction etc. larger than 1), the transfer pattern, that is, the master pattern, is enlarged xcex1-fold. This enlarged master pattern is divided into for example a x xcex1 number of patterns of master masks. If the magnification rate is 1/5 (xcex1=5), 5xc3x975=25 master masks are provided. As a result, since the patterns formed on the master masks are parts of the master pattern enlarged xcex1-fold from the master pattern, the amounts of the drawing data of the patterns of the master masks are reduced to about 1/xcex12 of the past and the minimum line widths become xcex1-times the past. Therefore, the patterns of the master masks can be drawn in a short time with little drift and with a high accuracy using for example a conventional electron beam lithography system or laser beam lithography system. For example, since the drawing error of the lithography system is reduced to 1/xcex1 on the photomask, the accuracy of the master pattern is further improved. Further, once these master masks are produced, the patterns of these master masks can be transferred at a high speed on to the substrate of the photomask by the step-and-repeat system etc., so the production time when producing a plurality of photomasks can be greatly reduced compared with the method of drawing individually by the lithography systems.
When there is an error in part of the patterns of the master masks at the time of formation or a change occurs in part of the master pattern after production, it is sufficient to correct or remake only the master masks including the part with the error or the master masks including the changed part. Since there is no effect on the plurality of master masks as a whole, it is possible to deal with these cases at a high efficiency.
When producing a working mask using a plurality of master masks, however, the plant producing the working mask stores a large number of master masks for dealing with all of the working masks to be produced. Therefore, when producing a working mask, the operator (worker) etc. has to set a plurality of master masks used in the production process (mask exposure apparatus) taking into consideration conditions such as at what position on the mask substrate (blank) of what layer of what product is being exposed. This work is extremely complicated resulting in easy occurrence of work errors.
Further, the various exposure conditions (exposure time, focus position, blind size, illumination conditions, shot XY magnification, etc.) differ for each master mask, so it is necessary to input and designate the respective exposure conditions or, when correcting the various error, input and designate the correction information for the same (correction values for deformation accompanying support of master mask, distortion of projection exposure apparatus, coma aberration, or other aberration, and deformation accompanying support of mask substrate etc.) The above problem was therefore extremely serious.
Therefore, an object of the present invention is to reduce the number of work steps and prevent the occurrence of work error when producing a photomask using a plurality of master masks.
Further, another object of the present invention is to produce a high accuracy photomask.
Note that in the following explanation, the present invention will be explained with reference to the reference numerals of members shown in the figures showing the embodiments, but the requirements of the present invention are not limited to the members given reference numerals and shown in the figure.
1. According to one aspect of the present invention, there is provided a method of dividing a pattern enlarged from a transfer pattern (27) into a plurality of patterns of master masks (Ri) and successively projecting and exposing images of the plurality of patterns (Pi) of master masks reduced by a projection optical system (3) on the surface of the mask substrate while stitching them, the method of exposure comprising steps of forming marks (M1, M2) including identification information for identifying one master mask from another master mask on each of the master masks, storing in advance mask information relating to the master masks corresponding to the identification information, detecting the mark of the master mask before exposure, and performing exposure in accordance with mask information corresponding to the identification information shown by the mark.
The master mask is formed with the mark including identification information for identifying the master mask from another master mask. The mark is detected before exposure and exposure is performed in accordance with mask information corresponding to the identification information shown by the mark, so by setting positional information showing the position on the mask substrate where the pattern of the master mask is to be transferred as the mask information, the operator etc. can simply extract the plurality of master masks required for production of a photomask and set them freely without identifying them in order to have the image of the pattern of the master mask transferred to the corresponding position on the mask substrate. Therefore, the operator etc. no longer has to set the masks considering conditions relating to the position such as for which position on the mask substrate the master mask is to be used for exposure, so the work becomes extremely easy and work errors become less frequent.
The mask information includes, in addition to the positional information showing the position on the mask substrate where the pattern of the master mask is to be transferred, for example correction information for canceling out deformation accompanying support of the master mask, aberration of the projection optical system, and deformation accompanying support of the mask substrate, the exposure time for the master mask, focus position, blind size, illumination conditions, shot magnification, and other exposure conditions, etc. Part or all of these may be included. By storing these correction information, exposure conditions, etc., intervention of an operator for the correction information or exposure conditions is no longer necessary, the number of work steps can be reduced, and input errors etc. are prevented. Further, even if the operator mistakenly sets different types of master masks, it is possible to identify this, so measures such as warnings can be devised and it is possible to eliminate the problem of exposure using master masks having no relation to the production of a working mask.
As the mark, it is possible to use a bar code mark or a spatial image measurement mark. When using a spatial image measurement mark, it is possible to use the mark for alignment of the master mask or correct various error based on the results of spatial image measurement. The mark may be detected in the state with the master mask held on the mask stage.
The above method forms a mark as identification information on the master mask. Corresponding mask information is stored in a storage device etc. This is particularly effective when there is a large amount of mask information. When the mask information is not that large, it is possible to directly form the mark including the mask information on the master mask. That is, it is possible to provide a method of dividing a pattern enlarged from a transfer pattern into a plurality of patterns of master masks and successively project and expose images of the plurality of patterns of master masks reduced by a projection optical system on to the surface of a mask substrate while stitching them where marks including mask information relating to the master masks are formed on each of the master masks and exposure is performed in accordance with the mask information shown by the marks.
An exposure apparatus of the present invention for working the above method of exposure provided with a plurality of master masks (Ri) formed with marks (M1) including positional information indicating a transfer position on a mask substrate (4) to be processed, a mask magazine (16) for storing the plurality of master masks, a mask stage (2) on which one master mask selected from the mask magazine is placed, a projection optical system (3) for projecting a reduced image of a pattern of the master mask on the mask stage on to a mask substrate, a substrate stage (6) for positioning the mask substrate on a plane vertical to an optical axis of the projection optical system, a detection device (56) for detecting content of the mark of the master mask on the mask stage, and a control device (9) for exposure in accordance with positional information shown by the mark detected by the detection device.
According to the exposure apparatus of the present invention, by forming the master mask with the mark including positional information showing the transfer position on the mask substrate where the pattern of the master mask is to be transferred, detecting the mark on the mask stage, and performing exposure at a position in accordance with the positional information shown by the mark, the operator etc. need only randomly load the plurality of master masks required for production of a photomask in the mask magazine in order to get the images of the patterns of the master masks transferred to the corresponding positions on the mask substrate. Therefore, the operator etc. no longer has to set the masks considering conditions relating to position such as for which positions on the mask substrate the master masks are to be used for exposure, so the work becomes extremely easy and work errors become less frequent.
2. According to another aspect of the present invention, there is provided a method of exposure for irradiating exposure light on a plurality of master masks (Ri) formed by dividing an enlarged pattern of a transfer pattern (27), reducing a pattern image of the same for each master mask, and transferring the same on to a mask substrate (4) on which the transfer pattern is to be formed, the method of exposure comprising steps of detecting deformation information (dxi, dyi) of the mask substrate corresponding to a transfer position of a pattern image and adjusting at least one of the relative positional relationship between the pattern image and the mask substrate at the time of transfer of the pattern image and the projection characteristics of the pattern image based on the deformation information. In this case, an exposure amount of said mask substrate can be changed in accordance with a change amount of a line width of the pattern image for every said master mask. Note that in the description and claims of the present application, xe2x80x9cdetecting deformation informationxe2x80x9d includes finding deformation information by actual measurement or finding it by simulation (calculation).
When successively projecting and exposing patterns while stitching them, since the positional accuracy or shape accuracy of the connecting parts of the patterns transferred by the master masks has a major effect on the quality or reliability of the photomask which is produced, it is extremely important that the positional or shape accuracy of the pattern be high. The mask substrate to be exposed is supported by a predetermined supporting method at the time of exposure, but flexing occurs due to the substrate""s own weight in accordance with the support method. This flexing differs depending on the position (shot) on the mask substrate, so sometimes the pattern distorts, misalignment occurs at the stitched parts of the patterns, or other cases occur not allowing transfer of patterns with a high accuracy.
Therefore, the method of exposure of the present invention detects deformation information of the mask substrate and adjusts (deforms) the shape of a pattern image to be transferred in accordance with the flexing or other deformation of the mask substrate for transfer. That is, the shape of the projected image on the surface is adjusted to the desired shape in the state with the mask substrate deformed in this way. Due to this, it is possible to produce a high quality, high reliability photomask. As the deformation information, for example, it is possible to employ as a standard the information in the case of supporting the mask substrate theoretically flat. By making adjustment based on this deformation information, it is possible to form a pattern having a pattern shape close to the ideal pattern shape in the state with the photomask produced supported ideally flat. The deformation information, however, is not limited to the standard of the ideal shape. It is also possible to use deformation information based on the state supported on the mask stage etc. of the device exposure apparatus (exposure apparatus for producing microdevices) at which the produced photomask is used. By making adjustment based on this, it is possible to form a pattern having the desired pattern shape in the state supported on the mask stage etc. of the device exposure apparatus.
Further, it is possible to detect the identification information formed on the master mask to obtain deformation information of the mask substrate. By doing this, input of the deformation information becomes unnecessary, the number of work steps can be reduced, and occurrence of defective products due to input error etc. can be prevented. Further, it is possible to support the mask substrate at a plurality of points without chucking. As the deformation information, it is possible to use information including information relating to flexing of the mask substrate due to its own weight.
Note that when adjusting the relative positional relationship between the pattern image and the mask substrate at the time of transfer of the pattern image, it is possible to make adjustments by for example shifting or rotating the position of the master mask and/or mask substrate, shift the stepping position, change the scan speed, change the scan direction, etc. Alternatively, it is possible to adjust the optical characteristics by adjusting the projection characteristics of the pattern image, for example, the lens controller of the projection optical system projecting the pattern image.
An exposure apparatus of the present invention for working the above method of exposure is an exposure apparatus provided with an illumination optical system (1) for irradiating illumination light to a plurality of master masks (Ri) formed by dividing an enlarged pattern of a transfer pattern (27) and a projection optical system (3) for reducing a pattern image for each master mask and projecting it on a mask substrate (4) on which the transfer pattern is to be formed, further comprising a detection device (56) for detecting information on deformation of the mask substrate in accordance with a transfer position of the pattern image and an adjustment device for adjusting at least one of a relative positional relationship between the pattern image and the mask substrate and projection characteristics of the pattern image at the time of transfer of the pattern image based on the deformation information. In this case, it is possible to further provide a stage for supporting the substrate at a plurality of points without chucking.
According to the exposure apparatus of the present invention, since the shape of the pattern image on the mask substrate is adjusted (deformed) for transfer, when projecting and exposing the patterns of the plurality of masks successively on the substrate while stitching them, it is possible to improve the continuity (continuity in the case of connection in the direction along the lines in the case of for example line-and-space (L/S) patterns) and periodicity (periodicity of arrangement in the direction orthogonal to the lines in the case of for example L/S patterns) of the connecting parts of the patterns formed using one master mask and another pattern formed using another adjoining master mask. Due to this, it is possible to produce a high quality, high reliability photomask.
3. According to another aspect of the present invention, there is provided a method for transfer of a pattern on to a substrate by exposing the substrate by illumination light through a mask formed with the pattern, comprising supporting the substrate at a plurality of points without chucking and adjusting at least one of a relative positional relationship between the pattern and the substrate and transfer conditions of the pattern at the time of transfer of the pattern based on information relating to flexing of the substrate by its own weight corresponding to the transfer position of the pattern on the substrate. In this case; it is possible to employ, as transfer conditions of the pattern, imaging characteristics of the projection optical system for forming a projected image of the pattern on the substrate. Also, said pattern is divided to more than one to be formed as a different mask and an exposure amount of said substrate can be changed in accordance with a change amount of a line width of said pattern image when transferring the pattern image on said substrate for every said mask. In this case, said substrate becomes a working mask to be used in an exposure apparatus for device production and an optical type reduction projection exposure apparatus can be used for transferring said pattern image.
The exposure apparatus of the present invention for working the above method of exposure is an apparatus for transferring a pattern to a substrate by exposing the substrate by illumination light through a mask formed with the pattern, provided with a stage for supporting the substrate at a plurality of points without chucking and an adjustment device for adjusting at least one of a relative positional relationship between the pattern and the substrate and transfer conditions of the pattern at the time of transfer of the pattern based on information relating to flexing of the substrate by its own weight corresponding to the transfer position of the pattern on the substrate.
4. According to still another aspect of the present invention, there is provided an exposure method for irradiating illumination light on each of a plurality of masks and transferring the pattern image on a photosensitive layer on a substrate for every said mask, wherein an exposure amount of said photosensitive layer can be changed in accordance with a change amount of a line width of said transferred pattern image at the time of transferring said pattern image at a part of said plurality of masks. In this case, said substrate becomes a working mask to be used in an exposure apparatus for device production and an optical type reduction projection exposure apparatus can be used for transferring said pattern image.